1. Field of the Invention
The present invention is generally related to a marine propulsion system and, more particularly, to a marine propulsion system in which the drive unit is overmolded with a thin overmolded layer of a polymer material to protect the metallic drive unit from corrosion.
2. Description of the Prior Art
Those skilled in the art of marine propulsion systems are aware that metallic drive units are subject to corrosion, particularly when used in salt water. In order to inhibit corrosion of the metallic drive unit, the gear case and drive shaft housing are often painted with a corrosion inhibiting primer and at least one coat of paint. However, when the paint is scratched or gouged, the protective characteristic of the primer and paint coats can become severely degraded.
Those skilled in the art of polymer materials and overmolding techniques are well aware of many products on which a polymer overmolded layer is used to seal or protect a surface of an object.
U.S. Pat. No. 5,487,687, which issued to Idzikowski et al. on Jan. 30, 1996, discloses a midsection and cowl assembly for an outboard marine drive. The drive has a midsection between the upper power head and the lower gear case and has a removable midsection cowl assembly including first and second cowl sections. The midsection housing includes an oil sump in one embodiment and further includes an exhaust passage partially encircled by cooling water and partially encircled by engine oil for muffling engine exhaust noise. The midsection housing also has an oil drain arrangement providing complete and clean oil draining while the outboard drive is mounted on a boat and in the water wherein the operator can change oil without leaving the confines of the boat and entering the water.
U.S. Pat. No. 6,468,119, which issued to Hasl et al. on Oct. 22, 2002, describes a composite sterndrive assembly. The assembly is configured for utilization in an inboard/outboard power plant for a boat. The sterndrive assembly includes a central rigid core that is configured at an upper portion to be coupled to the stern of a carrying boat. A lower portion of the core is designed to accept a boat moving force generated by a water propulsion unit that is coupled thereto. A thin-walled housing is configured to be secured about a predominance of the centrally located rigid core. The housing has an outer surface that establishes an exterior of the sterndrive assembly and an inner surface directed generally toward the central rigid core. A portion of an exterior surface of the central rigid core is configured to cooperate with a corresponding portion of the inner surface of the thin-walled housing. These two portions, when in cooperative orientation one with the other, form a functional feature for the sterndrive assembly.
U.S. Pat. No. 5,656,376, which issued to Rafferty et al. on Aug. 12, 1997, describes composite and fairwater structures for marine vessels. In a marine vessel having a drive shaft that extends rearwardly from its hull, wherein the drive shaft has a coupling and a bearing assembly along its length that are supported by struts, which struts are also secured to the hull of the vessel, a coupling cover encompasses a coupling and is mounted adjacent to a bearing assembly. A fairwater encompasses the coupling cover and is attached to the bearing assembly for the strut associated therewith to define a chamber and a clearance space between the fairwater and the coupling cover for directing fluids therethrough and through the bearing assembly to lubricate such bearing assembly. Vanes are located on the coupling cover or on a separate support located within such chamber to enhance the fluid flow through the bearing assembly. A laminate for use in the structures of the coupling cover, the fairwater, struts, vanes and supports is disclosed, wherein the laminate includes a fiber-reinforced toughened epoxy resin layer sandwiched between a vibration-damping elastomer layer and a biocidal elastomer layer; the marine laminate material can be shaped and sized into a marine structure and exhibits desirable marine properties including cavitation-resistance, anti-fouling and vibration damping.
U.S. Pat. No. 5,011,583, which issued to Tanbara on Apr. 30, 1991, describes a corrosion prevention system for a marine propulsion system. A marine propulsion system of the type having a sacrificial anode for corrosion protection of the casing includes structure whereby the propeller is electrically insulated from the casing and the sacrificial anode. The structure includes spacers made of insulating materials, spacers having insulating coatings, or insulating coatings on the surfaces of the propeller or the propeller shaft. Electrical insulation of the propeller prevents unsightly and efficiency-reducing depositions on the propeller surfaces and reduces the required size of the anode.
U.S. Pat. No. 6,173,669, which issued to Staerzl on Jan. 16, 2001, discloses an apparatus and method for inhibiting fouling of an underwater surface. A marine fouling prevention system comprises two conductive surfaces and a device that alternates the direction of electric current between the two surfaces. The current is caused to flow through seawater in which the two surfaces are submerged or partially submerged. A monitor measures the current flowing from one of the two conduction surfaces and compares it to the current flowing into the other conduction surface to assure that no leakage of current of substantial quantity exists. The system applies a low magnitude current density, of approximately 0.10 to 0.50 milliamperes per square foot, for an extended duration of time of approximately 10 to 20 minutes. By alternating current direction between the two surfaces, both surfaces can be provided with sufficient chlorine gas bubbles to prevent marine growth from attaching to the surfaces.
U.S. Pat. No. 6,209,472, which issued to Staerzl on Apr. 3, 2001, discloses an apparatus and method for inhibiting fouling of an underwater surface. A system for inhibiting marine organism growth on underwater surfaces provides an electric current generator which causes an electric current to flow proximate the underwater surface. A source of power, such as a battery, provides electrical power to the electric current generator. The flow of current passes from the underwater surface through water surrounding the surface or in contact with the surface, and a point of ground potential. The point of ground potential can be a marine propulsion system attached to a boat on which the underwater surface is contained.
U.S. Pat. No. 6,547,952, which issued to Staerzl on Apr. 15, 2003, discloses a system for inhibiting fouling of an underwater surface. An electrically conductive surface is combined with a protective surface of glass in order to provide an anode from which electrons can be transferred to seawater for the purpose of generating gaseous chlorine on the surface to be protected. Ambient temperature cure glass (ATC glass) provides a covalent bond on an electrically conductive surface, such as nickel-bearing paint. In this way, boat hulls, submerged portions of outboard motors, and submerged portions of sterndrive systems can be protected effectively from the growth of marine organisms, such as barnacles. The electrically conductive surface generates electrons into the seawater in order to create chlorine gas at the surface which inhibits and discourages marine growth. The protective coating of glass inhibits the migration of metal ions from the electrically conductive surface into the seawater and therefore inhibits corrosive degradation as a result of galvanic action.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
It is known that various types of coatings, such as paint, can be used to protect surfaces of components that would otherwise be subjected to corrosive attack because of the environment in which they are used. It is also known that composite structures can be attached to the external surfaces of marine drives, such as the systems described in U.S. Pat. Nos. 5,487,687 and 6,468,119. It would be significantly beneficial if an overmolded layer could be quickly and efficiently applied to a marine propulsion system that provides a rugged protective overmolded layer that is more durable than paint and more inexpensively applied than preformed housing elements that are later attached to the marine drive.